Personal dosimeter on the base of radiation integrated circuit.

ABSTRACT

This invention provides a radiation dosimeter and new method of operation which comprise two types of the metal-oxide-semiconductor field-effect transistor (MOSFET) circuits allowing to amplify the threshold voltage changes due to radiation and provide temperature compensation. The first type dosimeter is a radiation integrated circuit (RADIC) which includes two radiation field-effect transistors (RADFET) and two MOSFETs, integrated into the same substrate. The second type of radiation circuit includes two RADFETs, integrated into the same substrate, and two resistors. The amplification of the threshold voltage change is achieved by using amplification principles of an MOSFET inverter. In both cases, under the ionizing irradiation, the gate of first RADFET is forward biased and the gate of second RADFET is biased off. In the reading mode the amplified differential threshold voltage change is measured. The increased radiation sensitivity allows to measure of the milli-rad doses. The temperature effect and drift is substantially eliminated. These radiation integrated circuits can be used as a personal dosimeter in the nuclear, industrial and medical fields.

REFERENCES CITED Patent Documents

-   Canadian Patent 1,204,885 5/1986 Ian Thomson

OTHER PUBLICATIONS

-   B. O'Connell, A. Kelleher, W. Lane, L. Adams, <<Stacked RADFETs for    Increased Radiation Sensitivity>>, IEEE Tran. Nucl. Sci. Vol. 43,    N3, June 1996, pp. 481-486.-   V. Polischuk and G. Sarrabayrouse, <<MOS ionizing radiation    dosimeters: from low to high dose measurement>>, Radiation Physics    and Chemistry, Vol. 61, 2001, pp. 511-513.-   G. Sarrabayrouse, D. Buchdahl V. Polischuk, S. Siskos, <<Stacked MOS    ionizing radiation dosimeters: potentials and limitations>>,    Radiation Physics and Chemistry, Vol. 71, 2003, pp. 737-739.-   R. H. Crawford, “MOS FET in Circuit Design”, New York: McGraw-Hill.    1967.

BACKGROUND OF THE INVENTION

Metal-oxide-semiconductor dosimeters are MOS field-effect transistorswith a specially processed gate insulator in order to make it radiationsoft.

There are presently conventional personnel dosimeters such as thermalluminescent devices. Such devices use a small crystal of CaF₂ or LiFwhich traps the electrons and holes produced by the ionizing radiation.When heated, light is emitted from the crystal due to the emptying ofthe traps and this light is related to the accumulated dose.

A MOSFET dosimeter as commonly known as a radiation field-effecttransistor (RADFET) measures a shift in the threshold voltage of RADFETcaused by radiation.

Radiation field-effect transistors are their numerous advantages withrespect to thermal luminescent devices: low cost, small size and weight,robustness, accuracy, large measurable dose range, real-time or delayeddirect reading, information retention, possibility of monolithicintegration with other sensors and/or circuitry capable of performingmeasurement, signal conditioning and data processing.

Canadian Pat. No. 1,204,885 which issued May 20, 1986 to Ian Thomsondiscloses a radiation dosimeter comprising a pair of silicon insulatedgate field effect transistors (IGFET) by measuring the differentialthreshold between two IGFET sensors exposed to the same radiation, inwhich one is biased into its conducting region, and the other is biasedeither off or to a conducting level less than first. These dual IGFET'sdosimeter offer a sensitivity about 2 mV/cGy for case in which the gatebias is equal to 3 volts, or about 5 mV/cGy for the case in which thegate voltage is greater than 10 volts. The temperature sensitivity ofthe dual IGFET sensor has been found to be smaller than 0.1 mV/° C. Overthe military temperature range −20° C. to +50° C., a 70° difference,ΔV_(T)=7 mV or 1-3 cGy.

The problem associated with this prior art device is that it is notenough sensitive dosimeter for use by personnel workers in the medical,nuclear and industrial field. The personal dosimeter should have asensitivity of approximately 0.010 cGy (Rad).

B. O'Connell, A. Kelleher, W. Lane, L. Adams in a paper entitled<<Stacked RADFETs for Increased Radiation Sensitivity>> published inIEEE Tran. Nucl. Sci. Vol. 43, N3, June 1996 has demonstrated theradiation sensitivity of 80 mV/cGy by stacking of 15 individual RADFETson the same chip.

V. Polischuk and G. Sarrabayrouse in a paper entitled <<MOS ionizingradiation dosimeters: from low to high dose measurement>> published inthe revue of Radiation Physics and Chemistry, Vol. 61, No 3-6, 2001presented a stack-connected RADFET configuration with RADFETs having avery thick gate oxide of 1.6 μm. To increase the sensitivity and theminimum measurable dose up to 14 transistors have been stacked. Withthis the output voltage before irradiation was 18V. A sensitivity ashigh as 90 mV/cGy has been obtained.

Both teams claimed about possibility to measure milli-Rad doses.However, stacked RADFETs exhibit a number of problems which limit theiruse in personal dosimeters. The problem is that single RADFET has acertain temperature coefficient. The metal oxide semiconductorfield-effect transistor device has a temperature threshold voltagedependence that needs to be accounted for in order that only radiationinduced shift in threshold voltage is measured by dosimeter. For stackedRADFETs the temperature sensitivity increases more than N times (N isnumber RADFETs in stacked) than single one.

The inventor has measured the temperature sensitivity of stacked RADFETsmade by B. O'Connell's team by using their RADFETs dosimeters. Thetemperature response is 70 mV/° C. for 15 MOSFETs in stacked for smallconstant current mode of 10 μA. If the reading temperature is controlledas +/−1° C., the minimum measured dose is about 5 cGy or 5 Rad.

G. Sarrabayrouse, D. Buchdahl, V. Polischuk, S. Siskos in a paperentitled <<Stacked MOS ionizing radiation dosimeters: potentials andlimitations>> published in Radiation Physics and Chemistry, Vol. 71,2003, pp. 737-739. proposed to reduce temperature sensitivity of stackedRADFETs by measuring stacked RADFETs at the Minimum TemperatureCoefficient (MTC) point. Indeed this paper presents only the computersimulations. The temperature sensitivity at MTC point and thresholdvoltage drifts were not measured as well.

Another problem of stacked RADFET is its high output voltage which insome cases is about 18 volts. So it is difficult to amplify the smallchanges of threshold voltages, caused by radiation, by using operationalamplifiers.

In the present invention in order to increase radiation sensitivity weused the amplification principles of MOSFETs inverters described in thebook of R. H. Crawford, “MOS FET in Circuit Design”, New York:McGraw-Hill. 1967.

It is therefore an object of the present invention to provide aradiation integrated circuit as a personal dosimeter having a milli-Radsensitivity and temperature compensation by applying amplificationprinciples of inverters.

SUMMARY OF THE INVENTION

This invention relates to a solid-state dosimeter measuring very lowdoses of ionizing radiation from 0.01 cGy to 2 cGy and more particularlyto radiation integrated circuit (RADIC) based on RADFET and MOSFETelements or to circuit based on RADFETs and resistors. Duringirradiation of any of RADICs the first RADET (left) in the referencecircuit is biased off and the second RADFET (right) in the invertercircuit is biased. Thus the threshold voltage of the second RADFETvaries with the dosage to a considerably greater degree than that of thefirst RADFET. During measurement the threshold voltage change of thesecond RADFET (right) is amplified by its inverter circuit. The outputvoltage change will be equal to the amplified differential thresholdvoltage change:

ΔU _(out) =A _(u) ΔU _(T)

Thus the present invention solves the problems of low radiationsensitivity of conventional dual IGFETs and stacked connected RADFETsdosimeters.

Second object of this invention is that this radiation integratedcircuit has minimum temperature effect and is relatively insensitive totemperature changes. This is achieved by measuring the differentialthreshold voltage from two RADFETs. To assure that temperature affectboth RADFETs equally the circuits with both RADFET and both MOSFETs orwith two RADFETs were fabricated in the same silicon substrate, i.e. inthe same chip. The gate oxide thickness of each RADFET is preferably 1μm.

BRIEF INTRODUCTION TO THE DRAWINGS

A better understanding of the invention will be obtained by reference tothe detailed description of the invention below, and to the followingdrawings, in which:

FIG. 1 is a dual IGFET's dosimeter ready for measurement of itsdifferential threshold voltage.

FIG. 2 is a reading configuration of stacked connected RADFETsdosimeter.

FIG. 3 is an inverter with a MOSFET as a load.

FIG. 4 is an inverter with a resistor as a load.

FIG. 5 is a schematic of the radiation integrated circuit for the basicembodiment of the invention in its configuration prepared to acceptirradiation.

FIG. 6 is a schematic of the radiation integrated circuit dosimeter forthe basic embodiment of the invention in the reading mode.

FIG. 7 is a schematic of the radiation circuit for the second embodimentof the invention prepared to accept irradiation.

FIG. 8 is a schematic of the radiation circuit dosimeter for the secondembodiment of the invention in the reading mode.

FIG. 9 shows the response of the radiation circuit as a function of theradiation dose of gamma-ray for the second embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 3 shows an inverter made of two MOSFETs, where T1 is a managingMOSFET transistor and T2 is a loading MOSFET transistor. Amplificationcoefficient of this inverter according to a prior art from the book ofR. H. Crawford referenced in background of the invention is

$\frac{U_{out}}{U_{in}} = {A_{u} = {{- \sqrt{\frac{\beta_{1}}{\beta_{2}}}} = {- \sqrt{\frac{\left( {{W/d_{ox}}L} \right)_{1}}{\left( {{W/d_{ox}}L} \right)_{2}}}}}}$

Where β is given as

$\beta = \frac{{\mu ɛ}_{o}ɛ_{ox}W}{d_{ox}L}$

FIG. 4 shows an inverter made of a MOSFET and a resistor, where T1 is amanaging transistor and R_(D) is a loading resistor. Amplificationcoefficient of this second inverter according again to a prior art fromthe book of R. H. Crawford is proportional to steepness or transitionconductivity and to the loading resistance:

$\frac{U_{out}}{U_{in}} = {A_{u} = {- {g_{m}\left( {R_{D}\left. r_{d} \right)} \right.}}}$

Where (R_(D)∥r_(d)) is an equivalent resistance of parallel connectedloading resistor R_(D) and dynamic drain resistance r_(d).

$\left( {{R_{D}\left. r_{d} \right)} = {{\frac{R_{D} \star r_{d}}{R_{D} + r_{d}}g_{m}} = {\sqrt{2{\beta }} \star \sqrt{I_{D}}}}} \right.$

FIG. 5 illustrates the basic embodiment of the present invention. TheRADFET Q2 has its gate G2, drain D2 and source S2 connected together andthey are connected to the common source S. The RADFET Q1 has its gate G1biased by the battery 4 and its drain D1 and source S1 are connected tothe common source S. Two MOSFETs Q3 and Q4 have their drains D3 and D4,gates G3 and G4, sources S3 and S4 connected to the common source S.

Both RADFETS Q1 and Q2 and both MOSFETs Q3 and Q4 are subjected to thesame ionizing radiation. The MOSFETS have thin oxide thickness less orequal than 100 nm and RADFETs have oxide thickness more or equal than 1μm. Because the radiation sensitivity is proportional to oxide thicknessthe MOSFET have very small sensitivity in comparison with RADFET. It hasbeen found than radiation sensitivity is much greater for the biasedRADFET Q1 than non-biased RADFET Q2.

FIG. 6 illustrates the basic embodiment of the present invention in thereading mode. It shows the same RADFETs Q1 and Q2 and the same MOSFETsQ3 and Q4 in a radiation integrated circuit (further RADIC1) prepared tomeasure the radiation dose. The sources S1 and S2 of the RADFETs areconnected together and grounded. The gate G2 of RADFET Q2 is connectedto its own drain D2 and this drain is connected also to the gate G1 ofRADFET Q1. The gate G3 and the drain D3 of the MOSFETs Q3 are connectedtogether. The gate G4 and the drain D4 of the MOSFETs Q4 are connectedtogether as well. The source S3 of MOSFET Q3 is connected to the drainD1 of the RADFET Q1 and the source S4 of the MOSFET Q4 is connected tothe drain D2 of the RADFET Q2. A power supply U_(dd) is connected to thedrains D3 and D4 of both MOSFETs. Both MOSFETs should be the same. BothRADFETs should be the same as well. Both MOSFETs and both RADFETs arefabricated in the same die. Thus both RADFETs should have the sametemperature variation characteristics, the same initial thresholdvoltage and the same oxide charges prior to irradiation. Both MOSFETsshould have also the same temperature characteristics and the samethreshold voltage. The RADFET Q1 and MOSFET Q3 are the inverter whichcan amplify the threshold voltage change of RADFET Q1. The thresholdvoltage amplification is given by the following equation:

$A_{u} = {- \sqrt{\frac{\left( {{W/d_{ox}}L} \right)_{RADFET}}{\left( {{W/d_{ox}}L} \right)_{MOSFET}}}}$

In case when the RADFETs parameters are as W1=1200 μm, L1=50 μm anddox=1 μm and the MOSFET parameters are as W2=20 μm, L2=2400 μm anddox=0.1 μm the threshold voltage amplification is 17.

The output voltage U_(out) is measured between the drains D1 and D2 ofthe RADFETs. Prior to irradiation, the voltage U_(out) is measured as afirst amplified differential threshold. After irradiation the outputvoltage U_(out) is measured again. The output voltage change ΔU_(out) isequal to the difference between the output voltage before and afterirradiation or to the amplified differential threshold voltage ΔU_(T)due to the dosage received:

ΔU _(out) =A _(u) *ΔU _(T)

The RADFET Q2 and MOSFET Q4 is the reference circuit which has the sametemperature and drift characteristics as the inverter. Thus thetemperature effect of this radiation integrated circuit is eliminated.

FIG. 7 illustrates the second embodiment of the present invention. TheRADFET Q6 has its gate G6, drain D6 and source S6 connected together andthey are connected to the common source S. The RADFET Q5 have its gateG5 biased by the battery 4 and its drain D5 and source S5 connected tothe common source S.

Both RADFETS Q5 and Q6 are subjected to the same ionizing radiation.RADFETs have oxide thickness equal or more than 1 μm. During irradiationRADFET Q5 is biased by the battery 4 and RADFET Q6 is biased off. ThusRADFET Q5 has considerably higher radiation sensitivity than RADFET Q6.

FIG. 8 illustrates the second embodiment of the present invention in thereading mode. FIG. 8 illustrates the same RADFETs Q5 and Q6 and the sameresistors 2 and 3 in the radiation integrated circuit 2 (further RADIC2) prepared to measure the radiation dose. The sources S5 and S6 of theRADFETs are connected together and grounded. The gate G6 of RADFET Q6 isconnected to its own drain D6 and this drain is connected also to thegate G5 of RADFET Q5. The resistor 2 is connected to the drain D5 of theRADFET Q5 and the resistor 3 is connected to the drain D6 of the RADFETQ6. A power supply U_(dd) is connected to both resistors 2 and 3.

Both RADFETs Q5 and Q6 should be the same and are fabricated in the samedie. Thus both RADFETs should have the same temperature variationcharacteristics, the same initial threshold voltage and the same oxidecharges prior to irradiation. The RADFET Q5 and resistor 2 are theinverter which can amplify the threshold voltage change of RADFET Q5.The amplification of threshold voltage change of RADFET Q5 is given bythe following equation:

$A_{u} = {{- \sqrt{\left( \frac{{\mu ɛ}_{o}ɛ_{ox}W}{d_{ox}L} \right){RADFET}}} \star \sqrt{I_{D}} \star \left( {R_{1}\left. r_{d} \right)} \right.}$

In case when the RADFETs parameters are such as W=4000 μm, L=40 μm andR1=1000 kΩ the amplification of threshold voltage change of the inverteris 15.

The RADFET Q6 and resistor 3 are the reference circuit for the inverterand it has the same temperature and drift characteristics as theinverter. Thus the temperature effect of this radiation circuit isminimized. The measured temperature sensitivity of RADIC2 is 0.5 mV/C.

The output voltage change ΔU_(out) is equal to the amplifieddifferential threshold voltage ΔU_(T) due to the dosage received.

The radiation sensitivity of this radiation circuit (S=ΔU_(out)/D) is240 mV/cGy for the case of biased voltage during irradiation Ubias=3.3V.Taking into account the measured temperature sensitivity the minimummeasured dose is about 0.01 cGy or 10 mRad when temperature iscontrolled as ±1° C.

FIG. 9 shows the experimental output voltage changes as a function ofthe irradiation dose of gamma-rays for radiation integrated circuit(RADIC 2).

Thus it may be seen than the radiation integrated circuits (RADIC 1 andRADIC 2) of the present invention provide a more sensitive and accuratedosimeter circuit than prior art dual IGFET dosimeter orstacked-connected RADFETS dosimeter.

1. A radiation dosimeter comprising a pair of radiation field effect transistors with thick oxide, each having a gate, a source, and a drain, and a pair of metal oxide field effect transistors, with thin oxide, each having a gate, a source, and a drain, all transistors integrated into the same substrate, means for applying a voltage to drain of each MOSFETs, means for forward biasing of the gate of second RADFET and zero biasing the gate of the first RADFET under the influence of ionizing radiation, means for measuring the amplified differential threshold voltage of radiation integrated circuit.
 2. (canceled)
 3. A dosimeter as defined in claim 1, means for connecting the drain with the gate of first MOSFET and the drain with the gate of the second MOSFET, means for connecting the drain of the first MOSFET and the drain of the second MOSFET with voltage supply, means for connecting the gate with the drain of the first RADFET, means for connecting the drain of the first RADFET with the gate of the second RADFET, means for connecting the source of the first MOSFET with drain of the first RADFET and the source of second MOSFET with the drain of second RADFET, means for reading amplified threshold difference between the drains of RADFETs to obtain an initial amplified differential threshold of voltage corresponding to accumulated radiation dose before irradiation.
 4. (canceled)
 5. A dosimeter as defined in claim 1 or 3 in which the RADFETs and MOSFETs have aluminum gates or polysilicon gates.
 6. (canceled)
 7. A dosimeter as defined in claim 1 in which RADFETs have oxide thickness equal to or greater than about 0.5 μm.
 8. A method of measuring ionizing radiation dosage comprising: (1) measurement of an initial amplified differential threshold voltage of radiation integrated circuit 1 (RADIC 1) comprising two RADFETs and two MOSFETs or radiation integrated circuit 2 (RADIC 2) comprising two RADFETs and two resistances; (2) forward biasing the gate of second RADFET and zero biasing of the first RADFET exposed to ionizing radiation for radiation integrated circuit 1 and 2; (3) measurement of the amplified differential threshold voltage after irradiation between two RADFETs where the amplification is given for radiation integrated circuits: ${\left. {{{\left. \mspace{79mu} a \right)\mspace{14mu} {RADIC}\; 1\text{:}\mspace{14mu} A_{U}} = {- \sqrt{\frac{\left( {{W/d_{ox}}L} \right){RADFET}}{\left( {{W/d_{ox}}L} \right){MOSFET}}}}}b} \right)\mspace{14mu} {RADIC}\; 2\text{:}\mspace{14mu} A_{U}} = {{- \sqrt{\left( \frac{\mu \; ɛ_{O}ɛ_{OX}W}{d_{OX}L} \right){RADFET}}} \star \sqrt{I_{D}} \star \left( {R_{1}\left. r_{d} \right)} \right.}$ (4) subtracting of the amplified differential threshold voltage measured after irradiation from that measured before irradiation ΔU _(OUT) =A _(U) *ΔU _(T)
 9. The dosimeter as defined in claim 1 in which the radiation integrated circuits 1 and 2 have the amplification of differential threshold voltage change equal 17 and 15 and these values can be between 2 and 100 as function of MOSFET, RADFET and resistor parameters. 